ATR is European (like BMW), de Havilland Q400 is North American (like Ford);
ATR is an (Airbus) airliner with props, Q400 is a Utility aircraft (Beaver/TwOtter/Carribou/Dash8/etc);
ATR has quality and finese, Q400 has muscle and performance.
ATR is a 'plastic Pug', Q400 is an impressive (but difficult) beast.